Examinations are carried out with the aid of computer tomographs in many medical problem situations. Examinations such as these are also carried out for test purposes in a number of machine construction fields, in particular in materials science and flight safety.
X-ray radiation is used in this case because these solid bodies, for example non-metallic bodies, can be partially penetrated, so that it is possible to obtain knowledge about the distribution of materials within the body being analyzed.
The use of X-ray radiation has the disadvantage that, beyond a certain dose, it can damage biological tissue. Therefore, particularly in medicine, it is desirable to keep the radiation dose required for a measurement low.
In order to verify X-ray radiation, it is known that this radiation can be absorbed by specific scintillation materials, with the energy in the absorbed X-ray quanta being converted to light. The number of photons produced per X-ray quantum is in this case generally approximately proportional to their quantum energy.
A photodiode converts the light to a current which is digitized by an analog/digital converter. Since the self-absorption of the light in the scintillation material frequently has molecules added to it, which cause the frequency of the light that is produced to be shifted, in order in this way to prevent self-absorption of the light that is produced.
Furthermore, specific semiconductor materials in which the incident X-ray radiation can produce charge carriers, are also known for verification of X-ray radiation. The number of charge carriers produced per X-ray quantum is in this case generally approximately proportional to their quantum energy.
The known detectors for verification of X-ray radiation make use of the effects described above. In this case, it should be noted that the known integrating detectors produce only one measured value per measurement. The light flashes or charges which are produced by the large number of X-ray quanta received in each measurement period are thus integrated up over the duration of the measurement period. The intensity of the received X-ray radiation (the number of received X-ray quanta of medium quantum energy per unit time) is then obtained by division of the value integrated up by the detector by the medium quantum energy per X-ray quantum, which can be determined stochastically.
Since the measurement X-ray radiation emitted for measurement purposes in computer tomography normally has a polychromatic spectrum, hardening effects must be taken into account in this context. When the measurement X-ray radiation emitted from a radiation source passes through a measurement object, the X-ray radiation is subject in some cases to severe suppression of low-energy components of its spectrum, depending on the materials it is passing through and on the length of the beam path through the materials. The scattered radiation is thus shifted in the same way as the medium quantum energy of the received X-ray quanta toward higher energies in the spectrum.
In order to verify the two-dimensional distribution and thus to create an image of the incident X-ray radiation, it is known for a large number of identical detectors to be combined to form a detector unit for detection of incident radiation, and for emission of corresponding image information. The detectors are in this case preferably arranged alongside one another on a plane, in the form of an array.
This results in a different value for the actual medium quantum energy per X-ray quantum, as a result of hardening effects for each detector in a detector unit as a function of the material distribution in the measurement object being analyzed. This actual value can be determined only approximately by means of stochastic methods. Particularly in areas in which different materials in the measurement object being analyzed are adjacent to one another (for example bone edges), the approximate determination of the medium quantum energy per X-ray quantum is subject to major errors, despite numerical corrections.
A further disturbance variable on the measurement of X-ray radiation by use of computer tomographs is the scattered radiation, which is pronounced to a greater or lesser extent depending on the measurement object being analyzed. The scattered radiation may make up several tens of percentage points of the emitted measurement X-ray radiation, depending on the spectrum of the emitted measurement X-ray radiation and on the nature of the measurement object being analyzed. This leads to a considerable deterioration in the contrast in the measurement result obtained from the detectors in the detector unit.
A scattered radiation grid, through which only X-ray quanta which are in a specific direction and which have a specific energy (and which are thus important for the measurement) can pass, is therefore provided upstream of the detector unit in known computer tomographs. The scattered radiation grid generally has a specific collimator system in the form of a lamella arrangement, so that X-ray quanta of the emitted measurement X-ray radiation which strike the lamella walls are also absorbed.
The provision of a scattered radiation grid accordingly means that a certain percentage of the radiation quanta of measurement X-ray radiation which is emitted for measurement purposes is absorbed in the scattered radiation grid, and can thus no longer be detected by the detectors.
In consequence, the intensity of the radiation emitted for measurement purposes must be increased appropriately, owing to the scattered radiation grid.
In medical applications, this unavoidably leads to an increased patient dose. Furthermore, the scattered radiation can frequently also not be sufficiently well suppressed by the provision of a scattered radiation grid.